1. Field of the Invention
The present invention relates to a composite porous membrane and a method for producing the same, and more particularly to a composite porous membrane capable of carrying a proton-conducting polymer to form a fuel cell electrolyte membrane and a method for producing the same.
2. Description of the Related Art
A direct methanol fuel cell contains an electrolyte-electrode joined assembly, obtained by forming an anode and a cathode on the surfaces of a proton-conducting electrolyte. The electrolyte-electrode assembly is sandwiched between a pair of separators to form a unit cell, and a plurality of the unit cells are stacked and tightened by a tie rod or the like, to produce a stack of the direct methanol fuel cell.
For example, NAFION (a perfluorosulfonic acid polymer membrane, available from DuPont) moistened with liquid water is used as the electrolyte conventionally. However, there is a demand for increasing the mechanical strength of the electrolyte to improve the durability in the above tightening step and during long-term operation of the fuel cell. In view of this demand, a composite membrane having an increased mechanical strength, obtained by impregnating pores (voids) of a stretched porous polytetrafluoroethylene membrane with NAFION, is proposed in Japanese Laid-Open Patent Publication No. 8-162132.
The stretched porous polytetrafluoroethylene membrane has a large number of minute nodes and linkages, and the linkages extend from the minute nodes and connect the minute nodes three-dimensionally. The minute nodes and the linkages are formed during the process of stretching and pore formation of a polytetrafluoroethylene sheet. It is reported that the membrane has a porosity of up to 98% by volume.
Further, a composite membrane, obtained by distributing a graft polymer having a sulfonic acid group in pores of a porous membrane, is also known.
The above polytetrafluoroethylene sheet having the remarkably large porosity is poor in rigidity, and thereby cannot be easily handled in the process of the stretching, pore formation, and impregnation with NAFION. Also the resultant composite membrane is poor in rigidity and thereby cannot be easily handled.
Further, such electrolyte membrane having low rigidity is often dimensionally changed due to creep or deformation in long-term operation of the fuel cell. In this case, various problems are caused, for example, the power generation capacity of the fuel cell is deteriorated, or a gas leaks from one electrode to the other electrode.
In a case where the pore diameters of the pores in the composite membrane containing the graft polymer are excessively large, it is difficult to occlude the pores by the graft polymer. For example, when a pore with a pore diameter of 10 μm is occluded by a polystyrene, the molecular weight of the polystyrene has to be several million or more. However, such a high-molecular-weight polymer cannot be easily produced by graft polymerization. When the pores are not sufficiently narrowed, disadvantageously a gas leaks from one electrode to the other electrode, and the power generation capacity of the fuel cell is deteriorated.
In addition, a known method for producing a porous membrane contains the steps of dissolving a polymer in a solvent in which the polymer shows a high solubility (hereinafter also referred to as a good solvent), casting the resultant solution on a substrate such as a glass plate to form a thin membrane, soaking the thin membrane in a solidifying bath of a solvent in which the polymer shows a low solubility (hereinafter also referred to as a poor solvent) to replace the good solvent by the poor solvent, and thereby causing the phase separation to solidify the polymer. However, the polymer is shrunk in the process of solidifying the polymer and drying the solidified polymer. Therefore, the porosity of the porous membrane is at most 60% to 70% by volume, and the membrane has only a small number of pores for carrying the proton-conducting polymer.